Tonight's spooky outing for the Doctor (Peter Capaldi) and Bill (Pearl Mackie) didn't just give us goosebumps, it also left us with a whole heap of questions.

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"Why would Bill give up travelling in time and space to go flat-hunting?" was just the first of many queries posed by 'Knock Knock', the Doctor Who debut of Doctor Foster writer Mike Bartlett. Here's a few more...

1. Who's in the Vault?

The clues are piling up. Whoever's trapped inside the mysterious Vault that the Doctor has sworn to protect, it's clearly someone he has affection for.

Not only did he give the Vault's occupant a gift in the form of a piano, but our Time Lord's eager to spend time, and share a takeaway meal, with them. There are also indications that whoever's inside was once a fellow traveller in time-and-space. "I know you miss it all," the Doctor says. "But I'm stuck here too. We're both prisoners."

That doesn't mean that whoever's inside the Vault is a nice person, though – they seem thrilled by the idea of "young people [getting] eaten", which is concerning to say the least.

We've got our theory as to who's in there...

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What do you reckon?

2. Who is Harry's other grandfather?

Harry (Colin Ryan) mentions how his grandad went "grey-packing" (that's backpacking for old people, apparently) with his boyfriend and got arrested for trying to steal a piece of the Great Wall of China.

Played by Ian Marter, Harry briefly travelled with the fourth Doctor (Tom Baker) in the 1970s, but found himself ill-suited to travelling in time and space and left the TARDIS for a quieter life.

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While the mention of the older Harry was cut from 'Knock Knock', we do know a little of what happened to the character. In a 2010 episode of Doctor Who spin-off The Sarah Jane Adventures, Sarah Jane (Elisabeth Sladen) mentions that he continued working as a doctor, saving "thousands of lives" with his vaccines.

3. Is the Doctor regenerating sooner than we think?

We've already seen the Doctor apparently regenerating in the trailer for this series, which is odd given that Capaldi isn't supposed to be leaving the show till Christmas.

In 'Knock Knock', he lets slip and mentions regeneration to Bill for the first time, rather awkwardly changing the subject when she wants to know more.

Why include this moment in the episode unless the topic's going to crop up again, while Bill's still around? If you believe the rumours that the Doctor's new friend might not be sticking around beyond this series, it's looking increasingly likely that we could be seeing a regeneration in some form before December 25.

6. How do the man-eating bugs help keep Eliza alive?

We'll be honest – the explanation behind tragic Eliza's fate, and the Landlord's devotion to keeping her alive, left us scratching our heads.

So the Dryads helped to heal an ailing Eliza (Mariah Gale), while also turning her into wood. Somehow, feeding unwitting tenants to the bugs helped sustain her further.

But why did the bugs turn Eliza into wood and eat everybody else? Why / how does their diet help to sustain her anyway? How did the Landlord learn that the house needed to be fed to keep his mother alive (prior to inviting the first lot of tenants to the house)?

And while we're at it, why did Eliza forget that the Landlord was her son? Old age? Dry rot in the brain?

7. Does the Doctor need to sleep?

"Sleep is for tortoises!" the Doctor tells Bill, repeating a quote he first used when he was Tom Baker in the 1976 adventure 'The Brain of Morbius'. So can Time Lords go without sleep, or do they need 40 winks like the rest of us?

Well... it's complicated. A series of mini-episodes known collectively as Night and the Doctor, and released on the Doctor Who: Complete Sixth Series DVD and Blu-ray box sets, saw the eleventh Doctor (Matt Smith) go on a series of solo adventures while his companions were asleep.

In 'Knock Knock', he suggests that Time Lords only need to rest "after a regeneration, or a big lunch" – and sure enough, both the fifth Doctor (Peter Davison) and the tenth (David Tennant) were seen taking a long kip after adopting a new face.

But in 2015's 'Sleep No More', the Doctor told Clara (Jenna Coleman) that he does sleep, just when she's not looking, and in 50th anniversary special 'The Day of the Doctor', he talks about having dreams.

It seems as though Time Lords can sleep, if required, it's just not something they do regularly, or that they need to do anywhere near as much as humans.